The shape of the scene background determines the perceived path of a moving object.

Studies of the perception of motion in three-dimensional scenes have provided extensive information about the effects of changes in the size, speed, and disparity of an object's image on the perception of the object's trajectory. The present study demonstrates that this perception is not determined primarily by the object's motion but by the shape of the background against which this motion is displayed. The effect of a scene background on judgments of the trajectory of a moving object was examined in 2 experiments with 33 observers. In the first experiment, observers judged whether the trajectory was concave or convex. In the second experiment, observers judged which of 2 displays, differing in curvature of the motion path and curvature of the background, depicted the more curved motion path. Judgments of sign of curvature and judgments of relative magnitude of curvature were determined almost entirely by the background curvature.